On Sublevel Set Estimates and the Laplacian

Abstract

Carbery proved that if $u:\mathbb{R}^n \rightarrow \mathbb{R}$ is a positive, strictly convex function satisfying $\det D^2u \geq 1$, then we have the estimate $$ \left| \left\{x \in \mathbb{R}^n: u(x) \leq s \right\} \right| \lesssim_n s^{n/2}$$ and this is optimal. We give a short proof that also implies other results. Our main result is an estimate for the sublevel set of functions $u:[0,1]^2 \rightarrow \mathbb{R}$ satisfying $1 \leq \Delta u \leq c$ for some universal constant $c$: for any $\alpha > 0$, we have $$ \left| \left\{x \in [0,1]^2 : |u(x)| \leq \varepsilon\right\}\right| \lesssim_{c} \sqrt{\varepsilon} + \varepsilon^{\alpha - \frac12} \int_{[0,1]^2}{\frac{|\nabla u|}{|u|^{\alpha}} dx}.$$ For 'typical' functions, we expect the integral to be finite for $\alpha < 1$. While Carbery-Christ-Wright have shown that no sublevel set estimates independent of $u$ exist, this result shows that for 'typical' functions satisfying $\Delta u \geq 1$, we expect the sublevel set to be $\lesssim \varepsilon^{1/2-}$. We do not know whether this is sharp or whether similar statements are true in higher dimensions.